1. Field of the Invention
The present invention is generally related to nuclear reactors and more particularly to thermionic reactors.
2. General Background
Thermionic reactors use thermionic conversion devices for the direct conversion of heat to electricity. Thermionic converters operate in the following general manner. Heating of an element, such as the cathode, serves to dislodge electrons from the lattice structure of the cathode material by increasing their kinetic energy. These electrons flow to another element, such as the anode, which is maintained at a lower temperature. By connecting the two electrodes to a load, an electrical current is established and current flows which operates an electrically powered device. The use of thermionic devices in combination with nuclear reactors is particularly advantageous for reactors designed for use in outer space due to the potential for mass reduction while maintaining equal power output.
Thermionic reactor design has been concentrated on two different approaches, excore thermionic concepts and thermionic fuel elements. Excore thermionic reactors typically utilize a cylindrical reactor core having a high conductivity nuclear fuel. UC.sub.2 in graphite is usually used as the fuel. The thermionic devices are located exterior of the core. Thermal radiation from the nuclear fuel causes planar thermionic diodes to generate electrical power. Nuclear fuel swells at the high temperatures required for efficient thermionic diode operation. Isolating the diodes from the fuel prevents the dimensional changes of the fuel from affecting the precise tolerances needed for efficient thermionic diode operation. The excore systems require the use of less developed fuels such as UC.sub.2 to prevent excessive fuel temperatures because the heat must be conducted through the entire core before reaching the thermionic diodes outside the core. Even with a fuel having a high heat conductivity, this type of reactor is limited to relatively low power levels to maintain acceptable fuel temperatures. The second type of thermionic reactor utilizes thermionic fuel elements (TFE's). The TFE is a cylindrical thermionic diode with nuclear fuel in the center of the diode. The fuel cladding becomes the emitter and must be designed to accommodate fuel swelling and chemical attack by the numerous chemical species generated during the fission process. This concept can use a more developed fuel such as UO.sub.2 because the heat conductive path from the center of the fuel pellet to the thermionic diode is short. This concept can also be scaled to larger power levels because the TFE becomes a modular fuel element. However, accommodation of fuel swelling while maintaining the precise tolerances required for efficient thermionic diode operation present significant development problems for this design.
A variation of the excore design utilizes an annular core with thermionic diodes around the outside and the inside of the annulus. This variation uses a thermionic heat pipe module (THPM). The THPM incorporates a cylindrical thermionic diode similar to the TFE but with a central heat pipe for heat rejection. The device provides both power conversion and waste heat removal in a single module. This configuration provides an advantage over other excore concepts because it uses UO.sub.2 fuel instead of carbide fuel but still presents many of the other disadvantages associated with excore concepts because of its limited size potential and its high fuel temperatures.
It can be seen that current approaches to thermionic reactor design have been unable to address certain restrictions such as size and the type of fuel that can be used. These reactors only produce electrical power. If propulsive thrust is needed, either electric thrusters must be used or a separate propulsion system provided. Both alternatives can result in significantly heavier space craft. The electric thruster can be heavier system because of its very low thrust level. The use of a separate propulsion system will be heavier because of the additional components required. The use of electric thrusters can also increase space craft mass because their low thrust requires additional propellant to overcome gravity losses.